Study of the biochemical-genetics of circadian rhythmicity in Neurospora crassa has been undertaken in the hope of discovering the cause of such rhythms in eukaryotic organisms. The ultimate rationale is to study the regulation of selected enzymes or nucleic acids to probe for causes of circadian variation in amount or activity. A practical problem still being investigated is the culture conditions and equipment necessary to grow Neurospora in large amounts so that substantial amounts of mycelium can be harvested at defined points in the circadian cycle for biochemical analyses. A chemostat- like culture in which CO2 production is measured and used for monitoring circadian rhythmicity is being studied. The use of mycelium harvested from large Petri-plate and baking- dish cultures for biochemical analyses has shown that several enzymes and nucleic acids are subject to circadian regulation. These experiments are being continued with the hope of showing by DNA-RNA hybridization that certain classes of mRNA molecules are made at certain times of the day. In addition, studies are being initiated with selected enzymes to determine whether the synthesis or activity of the enzymes is regulated in a circadian manner, and what aspects of that control might be open to circadian modulation.